This invention relates to an objective lens system having an attachment lens inserted thereto, or removed therefrom to change the focal length of the system while maintaining a fixed image plane, and, more particularly, to improvements in the zoom lens system with its relay lens having an air space in which the attachment lens is positioned.
To change the focal length of a fixed-focal-length lens, or to shift the focal length range of a zoom lens, there have been known various methods such as those of removably attaching an additional lens from the outside at an air space between the rear vertex of the lens system and the focal plane, of substituting a portion of the objective lens, in the case of the zoom lens, the relay lens for another one, of removably attaching an afocal length whose angular magnification is not the unity of plus and minus sign to the basic objective at the front thereof, and of constructing the relay lens of the zoom objective in the zoom lens form.
In application of such methods to the lens systems of large diameter and heavy weight such as for use in television cameras, however, the operation of detaching the objective from the camera body followed by reassembling them along with the additional lens is very time-consuming and tedious. In the case of the front attachment of afocal nature, it is further required to increase the diameter of the attachment with the resulting price being boosted. The last-named method requires to increase the necessary number of lens element in the zoom type relay lens. Thus, disadvantages are given such that the physical length of the lens system is increased and that the price is raised.
The present applicant has proposed an objective lens system in U.S. Pat. No. 4,015,895 where the relay lens of the zoom objective is divided into two parts in such a way that an afocal light path is established therebetween. By inserting or removing an afocal lens into or from this optical path, the focal length range is shifted while the back focal length is maintained unchanged. Another proposal has been made in U.S. Patent Application Ser. No. 692,518, now U.S. Pat. No. 4,240,697, where a portion of the relay lens is interchanged with another.
It is, however, found that the latter requires an operating mechanism of severer accuracy than does the former, and the structure of the mechanism is also complicated. As far as the mechanical aspect is concerned, the former is preferred to the latter. Even in the former, however, the special creation of an air space where the optical path becomes afocal in the relay lens leads to enhance the difficulty of lens design. On this account, the applicant of the present invention has attempted to impart a negative power into a magnification converting lens adapted upon attachment in the relay lens to shift the focal length range towards longer values in U.S. Patent Application Ser. No. 777,852, now U.S. Pat. No. 4,157,211. Thus, the degree of freedom in the lens design for aberrational correction and power distribution is increased.
There would be no problem for the negative attachment lens provided that the basic objective is of the type where the Petzval sum does not result in a large negative value when the attachment lens is inserted. In an objective lens system which tends to have a negative value of the Petzval sum as is general in the zoom lenses, the use of the negative attachment lens contributes to an increase in the value of the Petzval sum in the negative sense. Thus, the deterioration of the image quality due to the field curvature is intensified.